This invention relates to novel porous mullite articles, the manufacture thereof and to the use of the porous mullite articles as contact materials and catalyst supports. The invention relates also to procedures for controlling the pore size distribution, total porosity and surface area of mullite bodies obtained by calcining preformed bodies of clay to form mullite and silica and removing silica from the mullite by leaching.
The thermal conversion of kaolin clay to mullite is well known in the ceramics art. A high purity kaolin clay can theoretically be converted by high temperature calcination into about 64% weight percent mullite. The remainder is an amorphous or crystalline silica, depending on the temperature. The addition of various sources of alumina such as bauxite increases the amount of mullite that can be obtained from a given amount of kaolin, thereby reducing the amount of free silica. Similarly, mullite obtained from various other sources of silica and alumina, such as kyanite or bauxite, will not necessarily contain free silica.
The mullitization reaction that takes place when kaolin clay is uitilized as the sole source of silica and alumina may be represented by the following equation where the approximate chemical formula for kaolin (without the water of hydration) is given as Al.sub.2 O.sub.3.2SiO.sub.2, and the formula for mullite is 3Al.sub.2 O.sub.3.2SiO.sub.2 : EQU 3(Al.sub.2 O.sub.3.2SiO.sub.2).fwdarw.3Al.sub.2 O.sub.3.2SiO.sub.2 +4SiO.sub.2.
The term represented by 4SiO.sub.2 is the free silica generated as a result of the conversion to mullite.
Mullite is widely used in ceramic applications such as in the manufacture of refractory grains. For these applications, dense impervious products are needed and porosity is undesirable. See, for example, U.S. Pat. No. 3,642,505. It is known that a purified form of mullite can be obtained by calcining suitable clay such as kaolin, and extracting the silica with a strong base. See, for example, U.S. Pat. No. 2,536,122 and Japanese Patent Application No. 69 84,626 (CA81(10)53673a). It is my understanding that removal of the free silica in this fashion (or by reaction with sources of Al.sub.2 O.sub.3) to form additional mullite is practiced to improve the refractoriness of the resultant solid. In this regard, it is noted that the Al.sub.2 O.sub.3 --SiO.sub.2 phase diagram (Phase Diagrams for Ceramists, Amer. Cer. Soc. Ed., 1964, Diagrams 313-314) shows that pure mullite does not melt until about 1850.degree. C.; however, in the presence of free silica, melting begins at only about 1600.degree. C. Therefore, by eliminating free silica, the refractoriness of mullite is improved to an extent such that the melting point is about 250.degree. C. higher.
With regard to the prior practice of removing silica from mullite produced by calcining clay, U.S. Pat. No. 2,536,122 describes grinding the clay after calcination and before the extraction step. It is reasonable to conclude that when the resulting leached mullite grains are formed into refractory articles, porosity in the finished articles is reduced by addition of binders and also by sintering procedures conventionally used in manufacturing mullite products. To the best of my knowledge and belief, there has been no acknowledgment or appreciation of the fact that mullite purification by removal of silica from mullitized kaolin would change the struture of the mullite, resulting in a form of mullite of controllable pore structure to provide novel porous bodies.
Recent patent literature is abundant in disclosures of catalyst supports containing mullite but there is no indication that the mullite is of the unique type obtained by calcining kaolin and leaching silica. Further, I am aware of no prior art teaching how to obtain hard, shaped articles consisting essentially of porous mullite or how to control pore structure of such mullite in the micro, meso- and macro-size ranges.